1. Field of the Invention
The present invention relates to an optical scanner which performs scans by the scanning of an optical beam, and, in particular, to an optical scanning device having a structure in which a micro mirror which is supported by torsion bars is made to oscillate so as to cause the direction of an optical beam to change.
2. Background Art
In recent years, optical scanners which scan optical beams of laser light or the like have been used as optical instruments such as bar code readers, laser printers, and head mounted displays, or as the optical intake devices of input devices such as infrared cameras and the like. Optical scanners having a structure in which a micro mirror obtained via silicon micromachining technology is oscillated have been proposed for this type of optical scanner. For example, FIG. 22 shows an optical scanner having a silicon micro mirror which is disclosed in Japanese Unexamined Patent Application, First Publication No. H11-52278 (Patent document 1, referred to below as ‘Conventional technology 1’). This optical scanner is manufactured using silicon micromachining technology and is formed having an overall size of several millimeters square. A supporting substrate 1 is formed as a rectangular thick plate having a recessed portion 1a formed in a center portion thereof. A mirror 2 which is formed from a silicon thin film is internally supported inside this recessed portion 1a. Two torsion bars 3a and 3b which are formed integrally with the mirror 2 protrude from two ends thereof. Distal end portions of these torsion bars 3a and 3b are fixed to the supporting substrate 1, and are connected respectively to pads 4a and 4b. As a result, the mirror 2 is able to be swung between the planar direction of the mirror and a direction which is perpendicular to the mirror surface by the twisting of the torsion bars 3a and 3b. Impurity ions are implanted at least at peripheral areas or at the surface of the mirror 2 so as to become diffused over these areas. Alternatively, these areas may be covered by aluminum or silver or by a conductive organic thin-film resulting in these areas forming an electrode portion 5 which is electrically conductive.
In contrast, fixed electrodes 7a and 7b are located respectively at positions on both sides of the recessed portion 1a on the surface of the supporting substrate 1 via an insulator 6. These fixed electrodes 7a and 7b are formed by semiconductors or from a conductive material which is made from an organic material, and inner side edge portions of each of these fixed electrodes 7a and 7b are placed adjacent to the electrode portion 5 which is located at the edges on 2 sides of the mirror 2. Condensers are formed between the electrode portion 5 and the respective fixed electrodes 7a and 7b. 
If a predetermined voltage is applied between a pad 8a of the one fixed electrode 7a and the pads 4a and 4b of the torsion bars 3a and 3b, then this voltage is applied to the mirror electrode portion 5 which is connected to the pads 4a and 4b, and electric charges having mutually opposite polarities are accumulated on the surface of the fixed electrode 7a and the mirror electrode portion 5 so as to form a condenser. Static electricity then begins to work between the fixed electrode 7a and the mirror electrode portion 5, and the mirror 2 starts to rotate. Next, after the mirror 2 has returned to its original position, by then applying voltage between the fixed electrode 7b on the opposite side and the mirror electrode portion 5, the mirror 2 is again rotated, this time in the opposite rotation direction. By performing this type of operation repeatedly, the mirror 2 makes a swinging motion by repeating a motion of rotating between the positions of maximum rotation in the anticlockwise direction and the clockwise direction.
Moreover, as an optical scanner in which a micro mirror obtained by means of silicon micromachining technology is oscillated, the structure described in Japanese Unexamined Patent Application, First Publication No. H10-197819 (Patent document 2) is known (referred to below as ‘Conventional technology 2’).
As shown in FIG. 23, this optical scanner is provided with a plate-shaped micro mirror 1 which is used to reflect light, a pair of rotation supporting bodies 2 which are positioned on a straight line and support both sides of the micro mirror 1, a frame portion 3 to which the pair of rotation supporting bodies 2 are connected and which surrounds the periphery of the mirror 1, and a piezoelectric element 4 which applies translational motion to the frame portion 3. In addition, this optical scanner is structured such that the center of gravity of the mirror 1 is located at a position outside the straight line connecting together the pair of rotation supporting bodies 2.
When voltage is applied to the piezoelectric element 4, the piezoelectric element 4 is made to expand and contract, so as to vibrate in the Z axial direction. This vibration is transmitted to the frame portion 3. When the micro mirror 1 is made to undergo relative motion relative to the driven frame portion 3 and the vibration component in the Z axial direction is transmitted to the micro mirror 1, because the micro mirror 1 has a left-right asymmetrical mass component relative to the axis formed by the X axis rotation supporting bodies 2, rotational moment is generated in the micro mirror 1 centered on the X axis rotation supporting bodies 2. In this manner, the translational motion which has been applied to the frame portion 3 by the piezoelectric element 4 is transformed into rotational motion centering on the X axis rotation supporting bodies 2 of the micro mirror 1.
Moreover, as shown in FIG. 24, an optical scanning device is also described in Japanese Unexamined Patent Application, First Publication No. H10-104543 (Patent document 3, referred to below as ‘Conventional technology 3’). In this optical scanning device, beam portions 3 and 3 extend in mutually opposite directions from both sides of a movable portion 2 in a vibrator 1, and are connected to two arm portions 4 and 4 of a fixed portion 6. Piezoelectric thin films 5 and 5 are provided respectively on the arm portions 4 and 4 of the fixed portion 6, and these piezoelectric thin films 5 and 5 are driven by the same signal which includes higher order vibration frequencies.
The above-described optical scanner of Conventional technology 1 is manufactured to be several millimeters square using silicon micromachining technology, and the electrode portion 5 is formed on at least peripheral areas or on the surface of the mirror 2. In addition, the pads 4a and 4b are provided on the torsion bars 3a and 3b, and it is necessary to place the respective fixed electrodes 7a and 7b and pads 8a and 8b at positions on both sides of the surface of the supporting substrate 1 via the insulator 6.
In this manner, because the electrode portion 5 is formed on at least peripheral areas or on the surface of the mirror 2, and the pads 4a and 4b are formed on the torsion bars 3a and 3b, and the respective fixed electrodes 7a and 7b and pads 8a and 8b are formed at positions on both sides of the surface of the supporting substrate 1 via the insulator 6, the manufacturing of this optical scanning device is complex, and not only have the causes for possible failures increased, but the time required for manufacturing has also increased. Accordingly, there is a problem in that cost increases.
Moreover, in the optical scanner of the above-described Conventional technology 2, because a structure is employed in which translational motion applied to the frame portion 3 by the piezoelectric element 4 is transformed into rotational motion centering on the X axis rotation supporting bodies 2 of the micro mirror 1, it is necessary to shift the center of gravity position of the micro mirror 1 relative to the torsion bars.
Moreover, the device also needs to have a certain thickness not only in the X-Y axial directions, but also in the Z axial direction, so that it is difficult for this device to be manufactured with a slender thickness.
Moreover, the optical scanning device of the above-described Conventional technology 3 has the drawback that a large torsion angle cannot be formed in the movable portion 2.
Namely, if a piezoelectric film is formed in the two narrow cantilever beam portions which support the two torsion bars protruding from the frame portion, then the rigidity of this portion increases and vibration which is induced in the piezoelectric film is not transmitted efficiently to the torsion bars. As a result, the torsional vibration of the mirror is reduced. Moreover, unless the vibration characteristics of the vibration source portion formed by the two cantilever beam portions and the piezoelectric film which is formed thereon are matched precisely, then the vibration amplitude of the torsional vibration of the mirror becomes suppressed and, at the same time as this, torsion modes other than torsional vibration are superimposed thereon so that accurate laser beam scanning cannot be achieved. Furthermore, in order to increase the drive force for the mirror by increasing the surface area of the piezoelectric film portion, it is necessary to increase the width of the cantilever beam portions. Because of this, an unnecessary two-dimensional vibration mode is generated in the same cantilever beam portion, so that at the same time as the vibration amplitude of the torsional vibration of the mirror is restricted, a vibration mode other than the torsional vibration is superimposed thereon. As a result, there is a problem in that it is not possible to achieve accurate laser beam scanning. Moreover, because the width of the cantilever beams is restricted to a narrow width, the formation of the top portion electrodes which are used to drive the piezoelectric film formed on this portion is made more difficult because of the narrow width, so that problems arise such as the yield during production being greatly affected.
FIG. 25 shows the same case as that of Conventional technology 3, and shows a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from a frame portion. The drive efficiency of the mirror portion scan angle was checked by a simulation calculation. The surface where Y=0 was taken as a plane of symmetry, and half of this was used as a model.
FIG. 26 shows the torsion angle of a mirror having a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from the frame portion shown in FIG. 25. The drive voltage was set at 1V, while the characteristics of a PZT-5A which are typical parameters were used for the electrical characteristics of the piezoelectric body, while SUS 304 characteristics were used for the material of the scanner frame main body. The torsion angle of the mirror portion was small at only 0.63°.